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Surface Treatment

The Surface Treatment Of Die Casting Parts

Surface treatment of die casting parts is to improve the surface state of die-casting parts, thicken the surface oxide film or form a new surface mold, so as to improve the corrosion resistance, decoration of the surface of die-casting parts or achieve other important characteristics. Common surface treatment methods for die castings include chemical conversion treatment, anodizing treatment, electroplating and painting.

In order to obtain a satisfactory surface treatment effect, the die castings need to be pretreated before treatment, including mechanical pretreatment (polishing, polishing, sandblasting, etc.) and chemical pretreatment (degreasing, acid washing and alkali washing) Aluminum alloy die castings The electroplating can improve the decoration of die castings, improve surface hardness and wear resistance, improve lubricity, and improve surface conductivity and reflectivity. In actual production, unless there are special requirements, aluminum alloy die castings are generally rarely electroplated.

Aluminum alloy die casting products generally have surface treatments: powder spraying (ie powder spraying), baking paint, oil spraying, oxidation, sandblasting, electroplating, etc. According to the thickness and smoothness of the product surface treatment.

In the process of machining, transportation, storage, etc., the surface of die castings often has oxide scale, rust molding sand, welding slag, dust, oil and other dirt. In order for the deep layer to be firmly attached to the surface of the workpiece, the surface of the workpiece must be cleaned before painting. Otherwise, it will not only affect the bonding force and corrosion resistance of the coating and the metal, but also make the base metal even if there is a coating. It can also continue to corrode under the protection of the layer, causing the coating to peel off and affecting the mechanical properties and service life of the workpiece. Therefore, the surface treatment of the workpiece before painting is an important guarantee and measure to obtain a high-quality protective layer and prolong the service life of the product.In order to provide a good workpiece surface, the following points should be paid attention to in surface treatment:

  1. No oil and moisture
  2. No rust and oxide
  3. No adhesive impurities
  4. No acid and alkali residues
  5. The surface of the workpiece has a certain roughness

The Method Of Surface Treatment

  • Manual Machining:Such as scrapers, wire brushes or grinding wheels. The rust and oxide scale on the surface of the workpiece can be removed by hand, but manual machining is labor-intensive, low in production efficiency, poor in quality, and incompletely cleaned.
  • Chemical treatment:It is mainly to use acid-base or alkaline solution to chemically react with oxides and oil stains on the surface of the workpiece, so as to dissolve it in an acid or alkaline solution to remove rust, oxide scale and oil stains on the surface of the workpiece, and then use nylon. The finished brush roller or 304# stainless steel wire (the wire brush roller made of acid and alkali resistant solution can be cleaned to achieve the purpose.
  • Mechanical treatment method:It mainly includes wire brush roller polishing method, shot blasting method and shot peening method. The polishing method is that the brush roller is driven by the motor, and the brush roller rotates at a high speed on the upper and lower surfaces of the strip in the opposite direction to the movement of the rolling piece to remove the oxide scale. The brushed iron oxide scale is washed away by a closed circulating cooling water flushing system. Shot blasting is a method of using centrifugal force to accelerate projectiles and project them to the workpiece for rust removal and cleaning.
  • Shot peening is divided into shot peening and sandblasting:Surface treatment with shot peening has a strong impact and obvious cleaning effect. However, the treatment of thin plate workpieces by shot peening is easy to deform the workpiece, and the steel shot hits the surface of the workpiece (whether shot blasting or shot peening) to deform the metal substrate. Since Fe304 and FE203 have no plasticity, they peel off after being broken, and the oil film is related to it. Therefore, for workpieces with oil stains, shot blasting and shot peening cannot completely remove the oil stains.

Depending on the method used, surface treatment techniques can be divided into the following categories.

Electrochemical Method

This method uses electrode reaction to form a coating on the surface of the workpiece. The main methods are:

(1) Electroplating

In the electrolyte solution, the workpiece is the cathode, and the process of forming a coating on the surface under the action of an external current is called electroplating. The plating layer can be metal, alloy, semiconductor or containing various solid particles, such as copper plating, nickel plating, etc.

(2) Oxidation

In the electrolyte solution, the workpiece is the anode, and the process of forming an oxide film on the surface under the action of an external current is called anodization, such as anodization of aluminum alloys. The oxidation treatment of steel can be carried out by chemical or electrochemical methods. The chemical method is to put the workpiece into an oxidizing solution, and rely on chemical action to form an oxide film on the surface of the workpiece, such as the bluing treatment of steel.

Chemical Methods

This method is no current action, the use of chemical interaction to form a plating layer on the surface of the workpiece. The main methods are:

(1) Chemical Conversion Coating Treatment

In the electrolyte solution, the metal workpiece has no external current, and the chemical substances in the solution interact with the workpiece to form a coating on its surface, which is called chemical conversion coating treatment. Such as bluing, phosphating, passivation, chromium salt treatment, etc. on the metal surface.

(2) Electroless Plating

In the electrolyte solution, the surface of the workpiece is catalytically treated without external current. In the solution, due to the reduction of chemical substances, certain substances are deposited on the surface of the workpiece to form a coating process, which is called electroless plating, such as electroless nickel plating, Electroless copper plating, etc.

Thermal Machining Method

In this method, the material is melted or thermally diffused under high temperature conditions to form a coating on the surface of the workpiece. Its main methods are:

(1) Hot Dip Plating

The process of placing a metal workpiece into molten metal to form a coating on its surface is called hot-dip plating, such as hot-dip galvanizing, hot-dip aluminizing, etc.

(2) Thermal Spraying

The process of atomizing molten metal and spraying it on the surface of the workpiece to form a coating is called thermal spraying, such as thermal spraying zinc, thermal spraying aluminum, etc.

(3) Hot Stamping

The process of heating and pressing metal foil to cover the surface of the workpiece to form a coating layer is called hot stamping, such as hot stamping aluminum foil.

(4) Chemical Heat Treatment

The process of contacting and heating the workpiece with chemical substances, and making a certain element enter the surface of the workpiece at a high temperature is called chemical heat treatment, such as nitriding, carburizing, etc.

(5) Surfacing Welding

By welding, the process of making the deposited metal accumulate on the surface of the workpiece to form a welding layer is called surfacing, such as surfacing wear-resistant alloys.

Vacuum Method

This method is a process in which materials are vaporized or ionized and deposited on the surface of the workpiece in a high vacuum state to form a coating. Its main method is.

(1) Physical Vapor Deposition (PVD)

Under vacuum conditions, the metal is vaporized into atoms or molecules, or ionized into ions, and deposited directly on the surface of the workpiece to form a coating process, which is called physical vapor deposition. The deposition particle beam comes from non-chemical factors, such as evaporation Sputtering, ion plating, etc.

(2) Ion Implantation

The process of implanting different ions into the surface of the workpiece under high voltage to modify the surface is called ion implantation, such as boron implantation.

(3) Chemical Vapor Deposition (CVD)

Under low pressure (sometimes also at normal pressure), the process in which gaseous substances form a solid deposition layer due to chemical reactions on the surface of the workpiece is called chemical vapor plating, such as vapor deposition of silicon oxide, silicon nitride, etc.

Other Methods

Mainly mechanical, chemical, electrochemical, physical methods. The main method of which is.

(1) Coating

The process of applying paint (organic or inorganic) to the surface of the workpiece to form a coating is called painting, such as spray painting, brushing, etc.

(2) Impact Plating

The process of forming a coating layer on the surface of the workpiece by mechanical impact is called impact plating, such as impact galvanizing.

(3) Laser Surface Treatment

The process of irradiating the surface of the workpiece with a laser to change its structure is called laser surface treatment, such as laser quenching, laser remelting, etc.

(4) Super Hard Coating Technology

The technology of preparing superhard film on the surface of the workpiece by physical or chemical methods is called superhard film technology. Such as diamond film, cubic boron nitride film and so on.

(5) Electrophoresis And Electrostatic Spraying

  1. Electrophoresis, the workpiece is put into a conductive water-soluble or water-emulsified paint as an electrode, and forms a solution circuit with another electrode in the paint. Under the action of the electric field, the coating solution has dissociated into charged resin ions, the cations move to the cathode, and the anions move to the anode. These charged resin ions, together with the adsorbed pigment particles, are electrophoresed to the surface of the workpiece to form a coating, a process called electrophoresis.
  2. Electrostatic spraying, under the action of DC high voltage electric field, the atomized negatively charged paint particles are directed to fly to the positively charged workpiece, so as to obtain the process of paint film, which is called static spraying. Edit this paragraph for other taxonomies.
  3. The common application of wire brush is the cleaning of castings, including ferrous and non-ferrous; the cleaning of spatter and welding slag during welding; as long as the strength of the base material is enough to withstand brushing, dust, corrosion, and paint. Wire brushes produce a beautiful surface and can also be used to create decorative surfaces.

All of the above categories of divisions are not absolute. For example, surface treatment technology can also be divided into three major technologies: surface modification technology, thin film technology, and coating technology. The significance of surface treatment: it can effectively improve the hardness and wear resistance of the workpiece surface.